JETM6 UFO-PFlow links

This MR adds the PFOs associated to UFOs, so that the links can be used in tagging studies. @kgreif has tested the output to make sure that he is able to access the linked PFOs, and that things behave as expected.

This change is expected to increase the size of JETM6 by around 1%, as tested on a data and MC sample. This has been discussed with the conveners, who are happy with this.

Tagging @mswiatlo @mleblanc @camacho @brle @jcantero @tnobe @cdelitzs